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<p>Provides the default initial value to a new object.
</p>
<table id="toc" class="toc"><tr><td>
<div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="value_initialization.html%23Syntax"><span class="tocnumber">1</span> <span class="toctext">Syntax</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="value_initialization.html%23Explanation"><span class="tocnumber">2</span> <span class="toctext">Explanation</span></a></li>
<li class="toclevel-1 tocsection-3"><a href="value_initialization.html%23Notes"><span class="tocnumber">3</span> <span class="toctext">Notes</span></a></li>
<li class="toclevel-1 tocsection-4"><a href="value_initialization.html%23Example"><span class="tocnumber">4</span> <span class="toctext">Example</span></a></li>
<li class="toclevel-1 tocsection-5"><a href="value_initialization.html%23See_also"><span class="tocnumber">5</span> <span class="toctext">See also</span></a></li>
</ul>
</td></tr></table>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Syntax">edit</a>]</span> <span class="mw-headline" id="Syntax">Syntax</span>
</h3>
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<td> <span class="t-spar">T</span><code><b>();</b></code>
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<td> (1)
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<td> <code><b>new </b></code> <span class="t-spar">T</span> <code><b>();</b></code>
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<td> (2)
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<td> <span class="t-spar">Class</span><code><b>::</b></code><span class="t-spar">Class(...)</span> <code><b>:</b></code> <span class="t-spar">member</span><code><b>() {...</b></code>
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<td> (3)
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<td> <span class="t-spar">T</span> <span class="t-spar">object</span> <code><b>{};</b></code>
</td>
<td> (4)
</td>
<td> <span class="t-mark-rev t-since-cxx11">(since C++11)</span>
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<td> <span class="t-spar">T</span><code><b>{};</b></code>
</td>
<td> (5)
</td>
<td> <span class="t-mark-rev t-since-cxx11">(since C++11)</span>
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<td> <code><b>new </b></code> <span class="t-spar">T</span> <code><b>{};</b></code>
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<td> (6)
</td>
<td> <span class="t-mark-rev t-since-cxx11">(since C++11)</span>
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<td colspan="10" class="t-sdsc-sep">
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<td> <span class="t-spar">Class</span><code><b>::</b></code><span class="t-spar">Class(...)</span> <code><b>:</b></code><span class="t-spar">member</span><code><b>{} {...</b></code>
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<td> (7)
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<td> <span class="t-mark-rev t-since-cxx11">(since C++11)</span>
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<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Explanation">edit</a>]</span> <span class="mw-headline" id="Explanation">Explanation</span>
</h3>
<p>Value initialization is performed in three situations:
</p>
<div class="t-li1">
<span class="t-li">1,5)</span> when a nameless temporary object is created with the initializer consisting of an empty pair of parentheses <span class="t-rev-inl t-rev-inl-since-cxx11"><span>or braces</span><span><span class="t-mark-rev t-since-cxx11">(since C++11)</span></span></span>.</div>
<div class="t-li1">
<span class="t-li">2,6)</span> when an object with dynamic storage duration is created by a <a href="new.html" title="cpp/language/new">new-expression</a> with the initializer consisting of an empty pair of parentheses <span class="t-rev-inl t-rev-inl-since-cxx11"><span>or braces</span><span><span class="t-mark-rev t-since-cxx11">(since C++11)</span></span></span>.</div>
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<span class="t-li">4)</span> when a named variable (automatic, static, or thread-local) is declared with the initializer consisting of a pair of braces.
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<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td>
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<div class="t-li1">
<span class="t-li">3,7)</span> when a non-static data member or a base class is initialized using a <a href="initializer_list.html" title="cpp/language/initializer list">member initializer</a> with an empty pair of parentheses <span class="t-rev-inl t-rev-inl-since-cxx11"><span>or braces</span><span><span class="t-mark-rev t-since-cxx11">(since C++11)</span></span></span>
</div>
<p>In all cases, if the empty pair of braces <code>{</code>} is used and <code>T</code> is an aggregate type <span class="t-rev-inl t-rev-inl-until-cxx14"><span>that is not a class type with a default constructor</span><span><span class="t-mark-rev t-until-cxx14">(until C++14)</span></span></span>, <a href="aggregate_initialization.html" title="cpp/language/aggregate initialization">aggregate-initialization</a> is performed instead of value-initialization.
</p>
<p>If <code>T</code> is a class type that has no default constructor but has a constructor taking <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../utility/initializer_list.html"><span class="kw804">std::<span class="me2">initializer_list</span></span></a></span></span>, <a href="list_initialization.html" title="cpp/language/list initialization">list-initialization</a> is performed. 
</p>
<p><br>
The effects of value initialization are:
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<span class="t-li">1)</span> If <code>T</code> is a class type with at least one user-provided constructor of any kind, the <a href="default_constructor.html" title="cpp/language/default constructor">default constructor</a> is called.
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<td><span class="t-mark-rev t-until-cxx14">(until C++14)</span></td>
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<td>
<div class="t-li1">
<span class="t-li">1)</span> If <code>T</code> is a class type with no default constructor or with a user-provided default constructor or with a deleted default constructor, the object is <a href="default_initialization.html" title="cpp/language/default initialization">default-initialized</a>.
</div>
</td>
<td><span class="t-mark-rev t-since-cxx14">(since C++14)</span></td>
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<div class="t-li1">
<span class="t-li">2)</span> If <code>T</code> is an non-union class type without any user-provided constructors, then every non-static data member and base-class component of T is value-initialized
</div>
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<td><span class="t-mark-rev t-until-cxx11">(until C++11)</span></td>
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<div class="t-li1">
<span class="t-li">2)</span> If <code>T</code> is an non-union class type without any user-provided constructors, then the object is <a href="zero_initialization.html" title="cpp/language/zero initialization">zero-initialized</a> and then the implicitly-declared default constructor is called (unless it's trivial)
</div>
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<span class="t-mark-rev t-since-cxx11">(since C++11)</span><br><span class="t-mark-rev t-until-cxx14">(until C++14)</span>
</td>
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<td>
<div class="t-li1">
<span class="t-li">2)</span> If <code>T</code> is a class type without a user-provided or deleted default constructor (that is, it may be a class with a defaulted default constructor or with an implicitly-defined one) then the object is <a href="zero_initialization.html" title="cpp/language/zero initialization">zero-initialized</a> and then it is default-initialized if it has a non-trivial default constructor
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<td><span class="t-mark-rev t-since-cxx14">(since C++14)</span></td>
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<span class="t-li">3)</span> If <code>T</code> is an array type, each element of the array is value-initialized</div>
<div class="t-li1">
<span class="t-li">4)</span> Otherwise, the object is zero-initialized.</div>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Notes">edit</a>]</span> <span class="mw-headline" id="Notes">Notes</span>
</h3>
<p>The syntax <span class="t-c"><span class="mw-geshi cpp source-cpp">T object<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span></span></span> does not initialize an object; it declares a function that takes no arguments and returns <code>T</code>. The way to value-initialize a named variable before C++11 was <span class="t-c"><span class="mw-geshi cpp source-cpp">T object <span class="sy1">=</span> T<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span></span></span>, which value-initializes a temporary and then copy-initializes the object: most compilers <a href="copy_elision.html" title="cpp/language/copy elision">optimize out the copy</a> in this case.
</p>
<p>References cannot be value-initialized.
</p>
<p>All standard containers (<span class="t-lc"><a href="../container/vector.html" title="cpp/container/vector">std::vector</a></span>, <span class="t-lc"><a href="../container/list.html" title="cpp/container/list">std::list</a></span>, etc) value-initialize their elements when constructed with a single <code>size_type</code> argument or when grown by a call to <span class="t-c"><span class="mw-geshi cpp source-cpp">resize<span class="br0">(</span><span class="br0">)</span></span></span>.
</p>
<p>Since C++11, value-initializing a class without a user-provided constructor, which has a member of a class type with a user-provided constructor zeroes out the member before calling its constructor:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> A <span class="br0">{</span>
    <span class="kw4">int</span> i<span class="sy4">;</span>
    A<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span> <span class="co1">// user-provided ctor, does not initialize i</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> B <span class="br0">{</span> A a<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// no user-provided ctor</span>
 
<a href="../io/basic_ostream.html"><span class="kw1439">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> B<span class="br0">(</span><span class="br0">)</span>.<span class="me1">a</span>.<span class="me1">i</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span> <span class="co1">// value-initializes a B temporary</span>
                              <span class="co1">// leaves b.a.i uninitialized in C++03</span>
                              <span class="co1">// sets b.a.i to zero in C++11</span>
<span class="co1">// (note that B{}.a.i leaves b.a.i uninitialized in C++14, but for </span>
<span class="co1">//  a different reason: C++14's B{} is aggregate-initialization)</span></pre></div></div>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Example">edit</a>]</span> <span class="mw-headline" id="Example">Example</span>
</h3>
<div class="t-example">
<div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;string&gt;</span>
<span class="co2">#include &lt;vector&gt;</span>
<span class="co2">#include &lt;iostream&gt;</span>
 
<span class="kw1">struct</span> T1 <span class="br0">{</span>
    <span class="kw4">int</span> mem1<span class="sy4">;</span>
    <a href="../string/basic_string.html"><span class="kw1045">std::<span class="me2">string</span></span></a> mem2<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span> <span class="co1">// no constructors</span>
<span class="kw1">struct</span> T2 <span class="br0">{</span> 
    <span class="kw4">int</span> mem1<span class="sy4">;</span>
    <a href="../string/basic_string.html"><span class="kw1045">std::<span class="me2">string</span></span></a> mem2<span class="sy4">;</span>
    T2<span class="br0">(</span><span class="kw4">const</span> T2<span class="sy3">&amp;</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span> <span class="co1">// a constructor, but no default</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> T3 <span class="br0">{</span> 
    <span class="kw4">int</span> mem1<span class="sy4">;</span>
    <a href="../string/basic_string.html"><span class="kw1045">std::<span class="me2">string</span></span></a> mem2<span class="sy4">;</span>
    T3<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span> <span class="co1">// user-provided default ctor</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<a href="../string/basic_string.html"><span class="kw1045">std::<span class="me2">string</span></span></a> s<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// calls default ctor, the value is "" (empty string)</span>
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    <span class="kw4">int</span> n<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>     <span class="co1">// non-class value-initialization, value is 0</span>
    <span class="kw4">double</span> f <span class="sy1">=</span> <span class="kw4">double</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// non-class value-init, value is 0.0</span>
    <span class="kw4">int</span><span class="sy2">*</span> a <span class="sy1">=</span> new <span class="kw4">int</span><span class="br0">[</span><span class="nu0">10</span><span class="br0">]</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// array of 10 zeroes</span>
 
    T1 t1<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// no ctors: zero-initialized</span>
             <span class="co1">// t1.mem1 is zero-initialized</span>
             <span class="co1">// t1.mem2 is default-initialized</span>
<span class="co1">//    T2 t2{}; // error: has a ctor, but no default ctor</span>
    T3 t3<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// user-defined default ctor:</span>
             <span class="co1">// t3.mem1 is default-initialized (the value is indeterminate)</span>
             <span class="co1">// t3.mem2 is default-initialized</span>
 
    <a href="../container/vector.html"><span class="kw1067">std::<span class="me2">vector</span></span></a><span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span> v<span class="br0">(</span><span class="nu0">3</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// value-initializes three ints</span>
 
    <a href="../io/basic_ostream.html"><span class="kw1439">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> s.<span class="me1">size</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">' '</span> <span class="sy1">&lt;&lt;</span> n <span class="sy1">&lt;&lt;</span> <span class="st0">' '</span> <span class="sy1">&lt;&lt;</span> f <span class="sy1">&lt;&lt;</span> <span class="st0">' '</span> <span class="sy1">&lt;&lt;</span> a<span class="br0">[</span><span class="nu0">9</span><span class="br0">]</span> <span class="sy1">&lt;&lt;</span> <span class="st0">' '</span> <span class="sy1">&lt;&lt;</span> v<span class="br0">[</span><span class="nu0">2</span><span class="br0">]</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
    <a href="../io/basic_ostream.html"><span class="kw1439">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> t1.<span class="me1">mem1</span> <span class="sy1">&lt;&lt;</span> <span class="st0">' '</span> <span class="sy1">&lt;&lt;</span> t3.<span class="me1">mem1</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
    delete<span class="br0">[</span><span class="br0">]</span> a<span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>Output:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">0 0 0 0 0
0 4199376</pre></div></div> 
</div>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: See also">edit</a>]</span> <span class="mw-headline" id="See_also">See also</span>
</h3>
<ul>
<li> <a href="default_initialization.html" title="cpp/language/default initialization">default initialization</a>
</li>
<li> <a href="direct_initialization.html" title="cpp/language/direct initialization">direct initialization</a>
</li>
<li> <a href="copy_initialization.html" title="cpp/language/copy initialization">copy initialization</a>
</li>
<li> <a href="list_initialization.html" title="cpp/language/list initialization">list initialization</a>
</li>
</ul>

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